It is known that, under certain operating conditions, "knocking" will arise in internal combustion engines. Under "knocking", one understands audio frequency oscillations of the compressed fuel-air mixture which is triggered by a pulse wave. During these oscillations, the heat transfer is highly enhanced at the piston and cylinder walls of the combustion engine. This results in damaging thermal overloading of these surfaces. Knocking, therefore, is to be difinitely avoided. It is necessary, however, to provide means which early and reliably indicate knocking in order to utilize the possible working range of the combustion engine as extensively as possible. A plurality of sensors have been known or proposed therefor. For example, U.S. Pat. No. 4,275,586, GAST et al, assigned to the assignee of this application, to which German Patent Disclosure Document DE-OS No. 28 01 969 corresponds, describes a sensor in which a resonant system is formed with the aid of a piezoelectric oscillation element operating as a bending oscillator which can indicate knocking of the internal combustion engine. The mechanical effectiveness of this structure is not free from disturbances, particularly upon strong jolts which may occur in a motor vehicle.
It is further known to optically observe and measure the course of combustion in a combustion engine. For example, a spectrometric measuring system to examine combustion of a Diesel motor is described in "Motortechnische Zeitschrift", Vol. 39 (1978) page 385 in which a quartz window is supplied in the combustion chamber wall through which spectrometric observation of the course of combustion is possible. It is also known from German Patent Disclosure Document DE-OS No. 27 01 777 to arrange a light sensitive sensor in the combustion space on which a laser beam is impinged which previously was tangent to the terminal end of an injection nozzle in order to measure in this manner the injection time at injection valves. An optical analyzing apparatus is known from the "Rev. Tech. Automob.", 1979, pages 89, 90, which is screwed into the combustion engine at the position of the spark plugs in order to carry out calorimetric analysis of the fuel-air mixture. An arrangement was described in a workshop meeting "Kohlenwasserstoffemission" der Forschungsvereinigung Verbrennkraftmaschinen e.V. ("Hydrocarbon Emission" of the Research Society for Combustion Engines, registered society) on Oct. 10, 1979 in Aachen in a presentation by Spicher, in which the spreading of flames in the combustion space of the combustion engine is sensed by light guides and subsequently connected photodiodes, which light guides are screwed in the cylinder wall.